Medieval times means slightly different things around the world. In the UK it is the period between the arrival of William the Conqueror from France in 1066 and Henry VII, the first of the Tudor kings in 1485. In Europe, many of the things we know as medieval were already in place.
Medieval times was a time when lands were divided up into small kingdoms, principalities, dukedoms and so on, and when there was a strict way of dividing up the way of life into three: nobles, church and peasants. Nobles protected the people, the church looked after the souls and the peasants provided the food and other essentials for all. This system is known as the feudal system. It all revolved around the area of land called the manor. The manor was a workable unit of land controlled by a nobleman and worked by the peasants who lived on that land.
Medieval times were uncertain times, which is why many noblemen built castles to live in. Medieval times only really stopped when strong kings were able to control all of their land properly.